1. Field of the Invention
The invention relates to a magnetic resonance imaging method for reducing image errors in a magnetic resonance image formed from magnetic resonance signals from an object arranged in a steady magnetic field. The invention also relates to a magnetic resonance imaging device for reducing image errors in a magnetic resonance image formed from magnetic resonance signals from an object arranged in a steady magnetic field, which device comprises means for generating the steady field, means for generating RF electromagnetic pulses, and means for generating gradient magnetic fields superposed on the steady field.
2. Description of the Related Art
A magnetic resonance imaging method and device of the kind set forth are known from European Patent Application No. 0 212 526 which corresponds to U.S. Pat. No. 4,706,026, issued Nov. 10, 1987. The cited Application discloses a method of reducing image errors in magnetic resonance imaging, which errors are caused by periodic motion of the object, for example due to respiration. When magnetic resonance signals are acquired in known manner, for example by means of a so-called spin-warp sequence as disclosed in EP 0 212 526, during periodic motion of the object, image errors such as ghost errors and blurring occur in a magnetic resonance image derived from signal samples of the resonance signals. EP 0 212 526 also describes how such image errors can be reduced by selecting, in a pulse and gradient sequence, amplitudes of a phase encoding gradient in the pulse and gradient sequence in accordance with a predetermined diagram. Thus, EP 0 212 526 aims to reduce image errors which are caused essentially by a periodic variation of the magnetic resonance signals received. However, other sources of image errors also exist, for example varying external magnetic fields which disturb the steady magnetic field. These external fields may be caused by moving elevators in the vicinity of the MRI equipment, passing street cars or trains, etc. This means that the steady magnetic field fluctuates. Ultimately, after image reconstruction, the magnetic field disturbances will also give rise to image errors in the magnetic resonance image. Rigid motions of the object, without involving deformation of the object as occurring during respiration, also give rise to image errors. Furthermore, when the steady field is generated by means of a permanent magnet, drift phenomena could occur in the steady field due to temperature fluctuations. Slow drift phenomena of amplifiers in the transmission/receiving chain or of gradient amplifiers may cause similar problems.